The present invention relates to a system for vacuum depositing a material onto a sample having one or more surface recesses and, more particularly, for eliminating undesirable depositing angles of atoms on the sample.
In the manufacture of many electronic components, such as integrated circuits, there is a need to deposit metallic films on substrates. Materials such as copper may be deposited on ceramic or glass substrates and then etched or otherwise fabricated into the circuits and/or components.
In the field of plasma deposition, an atom may be displaced from the surface of a target connected to a cathode by a process called sputtering or sputter deposition. In this process, the target may be constructed of copper or another material. The cathode to which the target is attached is subjected to a high negative DC voltage in an inert atmosphere such as argon. The inert gas is ionized forming a plasma from which positive ions escape to bombard the exposed surface of the target and to dislodge by momentum transfer the atoms or clusters of atoms of the target material. It is this dislodging of the target atoms that is known as sputtering. By repeating this process, a number of these primarily neutral atoms move through the space in front of the target, in a relatively high vacuum, until they strike and condense on the surface of a receiver, known as a sample or substrate, which is generally in close proximity to the target. A coating of atomic or molecular layers of target material can thus be built on the substrate. The coating, which is generally less than 1 .mu.m, is called a thin film. It is generally sufficient for the metallization of integrated circuits.
The most commonly used plasma reactors have a target oriented such that the surface of the target is plane parallel to the surface of the sample on which sputtered atoms are to be deposited. It should be noted that atoms emitted from the target are distributed according to a cosine law. That is, the number of atoms ejected from the surface of the target is proportional to the cosine of the angle relative to the perpendicular at which they are ejected. Accordingly, atoms ejected from the target perpendicularly and received at the sample surface perpendicularly provide optimum atom deposition thereof.
Through-holes or viaduct holes (often called vias) are paths for electrical interconnections between a first-level conductive pattern and a second- or higher-level conductive pattern. In order to electrically connect circuits on different substrate levels to each other, precious metal (e.g., palladium) seeding and electroless metal deposition have been used to coat the walls of the vias, often followed by electroplating. Most recently, however, plasma technology has been applied to this problem.
The optimum orientation of target with respect to sample for purposes of depositing sputtered atoms on the surface of the sample has been found to be the worst orientation for coating via walls. This is due to the fact that the vias are generally oriented perpendicular to the plane of the substrate and, therefore, the plane of the target. Perpendicularly ejected target atoms pass directly through the holes and do not coat their walls.
U.S. Pat. No. 3,846,294, issued to Vossen, considers the problem of coating the interior walls of through-holes and attempts to solve it by disposing a substrate containing through-holes directly upon the surface of the target. The target material is sputter-etched from the surface of the target directly beneath the through-holes and is coated on the interior walls thereof.
Unfortunately, the amount of heat generated on the sample during the foregoing and similar processes exceeds safe limits for many, if not most, samples containing polymers and other organic material such as is used in circuit board manufacturing applications. Excessive heat causes difficulties, such as a loss of dimensional stability, polymer integrity and process reproducibility.
U.S. Pat. No. 3,361,659, issued to Bertelsen, teaches the use of a wire grid located between a cathode and an anode for use in a sputtering system. A variable potential is applied to the grid so that either an electric field-free region is created which prevents electron bombardment of the substrate or a small electric field is created in the vicinity of the substrate which induces sorption of gases into the growing film that is deposited on the substrate. The grid can be of positive potential with respect to the cathode to prevent negatively charged OH ions from contaminating the deposited film.
U.S. Pat. No. 3,856,654, issued to George, teaches the use of shields in an apparatus for sputtering thin films on a continuous procession of substrates. Trays of substrates are advanced within a sputtering chamber in a circular path and are incrementally advanced so that each substrate receives a uniform deposit of a thin film. An ionizable inert gas is introduced into the chamber, forced to strike the diffusion shields and scattered across the face of the cathode so that the resultant plasma is of substantial uniform composition. Shields are used in this above-identified reference to achieve uniform distribution of the inert gas ions on the substrate surface.
U.S. Pat. No. 3,904,503, issued to Hanfmann, teaches the use of a shield shaped to conform to plots of lines of constant thickness of the material deposited by a sputtering machine. The shield shades selected portions of the substrate and is withdrawn after a predetermined interval so that the uniformity of the material on the surface of the substrate is increased. Moreover, the area of the shield can be increased or decreased to accommodate changing conditions.
U.S Patent Nos. 4,301,192 and 4,383,495, issued to Plichta, et al, teach the use of fingers coated with ink that are inserted in through-holes and then withdrawn, thereby drawing the ink down into the through-holes and coating the walls thereof.
U.S. Pat. No. 4,416,755, issued to Ceasar, et al, discloses an apparatus and method for producing films on a substrate wherein a vacuum chamber has a plasma generator and a shield means to intercept stray or deflected ions. The presence of the shield minimizes sputtering of the chamber walls by the ion beam. A substance, such as carbon, with relatively low sputtering efficiency, is used to coat the stainless steel interior of the reactor chamber so that the resulting deposited film does not include impurities therefrom. The ion beam is a focused monoenergetic beam of positive ions. Screen and accelerator grid electrodes are used to control the beam energy by varying the bias voltage applied to the grid assembly. The grid assembly also serves as a radiation and mass barrier isolating the substrate from the plasma generating process.
U.S. Pat. No. 4,416,759, issued to Harra, et al, discloses a plasma reactor having blocking means which consists of a primary blocking shield and an ancillary blocking shield. The primary blocking shield intercepts atoms sputtered directly from the cathode target, whereas the ancillary blocking shield intercepts atoms sputtered from the cathode which have been redirected to travel underneath the primary blocking shield onto a region where no deposition is acceptable.
It would be advantageous to provide a system to coat the walls of surface recesses without the use of precious metal seeding, electroless deposition or other so-called wet processes.
It would also be advantageous to minimize heat generated on the sample during such a coating process.
It would further be advantageous to increase the ratio of wall deposition to sample surface deposition.
Moreover, it would be advantageous to allow sputtered atoms to bombard the sample and surface recesses therein at an angle relative to the plane of the sample, while eliminating undesirable, substantially perpendicular angles of atom bombardment.
Finally, it would be advantageous to provide a relatively simple, mechanical means to improve the ratio of sputter deposition into surface recesses such as through-holes and channels relative to deposition onto the upper surface of the sample.